Prostate cancer is the second most frequently diagnosed cancer and the sixth leading cause of cancer death in men. Brachytherapy has emerged as a definitive treatment for early stage prostate cancer. The procedure entails permanent implantation of small radioactive isotope capsules (seeds) into the prostate to kill the cancer with radiation.
Prostate brachytherapy is delivered with real-time transrectal ultrasound (TRUS) image guidance (FIG. 1). Typically, the TRUS probe 2 is translated and rotated by a mechanical stepper 4 in the rectum 6 with its displacement and rotation angle tracked by encoders on the stepper. Individual TRUS images of prostate contours are then compounded into a volume based on which an implant plan can be created and a radiation dose calculated. Finally, under real-time, intraoperative TRUS image guidance, the actual implants are delivered to the prostate 8 transperineally by needles 10 inserted through a template 12 that contains a rectilinear grid of guide holes. Success of this treatment depends on an accurate plan of radiation dosimetry and a precise delivery of the implant.
The intrinsic accuracy of a brachytherapy system is solely determined by a unique procedure called “calibration”, where a spatial registration between the coordinate systems of the TRUS and the template must be established prior to the implant procedure. Inaccurate system calibration causes faulty needle and radiation source placement, which may directly contribute to dosimetry errors, toxicity, and treatment morbidity.
In current practice, brachytherapy system calibration is a laborious, three-stage process:
Stage 1: An operator (typically a medical physicist) ascertains whether the TRUS image truthfully represents the size and shape of scanned objects and whether a series of individual images can be correctly stacked in space to reconstruct an accurate TRUS volume. For these purposes, artificial objects (phantoms) are employed with known geometry suspended in tissue-mimicking gel (to match the speed of sound in tissue). Phantoms are made commercially for these tasks; e.g., the industry-standard Brachytherapy Phantom CIRS 045 manufactured by Supertech, Elkhart, Ind. (U.S. Pat. No. 5,196,343). The operator scans the phantom, measures the distance, size, shape, and volume of the visible 2D and 3D features in the TRUS images, and then compares them to the known geometric specifications provided by the phantom manufacturer. Such measurements are conducted manually using rulers and calipers, either on the display of the ultrasound scanner, or on the printed TRUS images.
Stage 2: The operator calculates the relative spatial transformation between the coordinate frame of the TRUS images and the coordinate frame of the template. In the usual workflow, the operator mounts the template and the TRUS probe on a stand, dips the probe in a water tank, inserts needles through the template into the tank under TRUS imaging, marks the needle tips in the images, and calculates the transformation between the TRUS and template coordinate frames.
Stage 3: For some TRUS scanners that offer the ability to superimpose a square grid of coordinates on the real-time image, the overlaid grid lines must be aligned with the grids on the template. This is typically done by using eyesight and manually adjusting the scanner's setup. The user dips needles through the template into a water tank and then turns the knobs on the TRUS scanner until the grid lines appear to coincide with the artifacts created by the needles.
There are a number of technical elements in the calibration workflow that can lead to substantial bias and error in the final result:                The needles may be bent, therefore the segmented tip positions do not truthfully correspond to the physical locations of the template holes, which leads to erroneous template-TRUS registration;        The needle tip may be inaccurately segmented, especially when beveled implant needles are used;        The coordinates of the needle holes may be erroneously recorded;        The depth of the needle may be erroneously measured and recorded;        The number of needles used may be inappropriate; typically, too few needles are used;        The distribution of needle positions may be inappropriate, introducing bias if needle tips do not properly surround the location of the prostate;        The speed of sound in water is different from the speed of sound in human tissue, which can result in significant distance measurement errors in the TRUS image.        
Overall, the procedure is laborious, more qualitative than quantitative, and involves a great deal of eyeballing and subjective judgments by the operator.
Furthermore, the calibration is performed only periodically (primarily due to the inefficiency of the procedure), mostly outside the operating room, with the assumption that calibration parameters remain valid over time. In reality, however, calibration parameters may change during storage, transportation and setup of the equipment.
Perhaps most critically, the system calibration errors are difficult to detect during the procedure so the brachytherapist has no assurance whether the system is functioning correctly in the operating room. There is no validation mechanism in the current procedure to verify and ascertain the calibration accuracy in the operating room.
Finally, brachytherapy calibration, with its current practice, is a major recurring cost for care facilities, consuming manpower, time and money. One must book a calibration room, decommit the TRUS unit from clinical use, transport the equipment, prepare supplies (needles, water tank, etc.), set up the system, collect and process data, and log, analyze and document the results, dispose all used supplies, pack away the brachytherapy system, and return the TRUS scanner to the clinic. This workflow needs to be repeated from time to time.